The invention relates to a method of producing a screen on a panel for a color display device, which screen comprises a dotted structure of apertures in a black matrix and electroluminescent material in said apertures in which method a photosensitive material on the panel is exposed to light emitted by a point source and passed through a segmented lens and a mask, the segmented lens comprising an array of facets with boundaries between them, at least two of the facets having a top surface which is inclined at different angles and simultaneously changes the relative position of the segmented lens and the panel in a direction oblique to the boundaries of the facets during exposure of the photosensitive material, the extent and direction of changing the relative position of the segmented lens and the panel being such that, in moving from one extreme position to another extreme position, an image of a first facet on the panel occupies substantially an extreme position previously occupied by an image of a second facet obliquely adjacent to the first facet.
The invention also relates to a screen produced by using such a method and to a color display device provided with such a screen.
A method of producing a screen for a color display device as described above is disclosed in U.S. Pat. No. 4,866,466. The method according to this specification describes an exposure process for manufacturing screens for color display devices, like cathode ray tubes.
On the inside of the panel, which is the glass faceplate, cathode ray tubes (CRTs) are provided with the so-called screen. This screen has a black matrix structure and electroluminescent material in the apertures left free by the black matrix. The structure of the black matrix in most common CRTs is either a dotted structure or a line structure. This structure is produced by exposing a photosensitive material that is deposited on the inside of the panel and by using an exposure system and the shadow mask serving as the color selection means in CRTs. For exposing line-type CRTs, an exposure system with a continuous exposure lens can be used. However, for dotted-type CRTs, it is common practice to apply a segmented exposure lens in order to have enough degrees of freedom to obtain a dotted structure on the screen that fulfills the requirements regarding good landing properties. Landing in a CRT is the quality that defines how well the electron beams hitting the screen coincide with the corresponding electroluminescent material.
After the black matrix layer has been applied on the inside of the panel, another photosensitive process is used for applying the electroluminescent materialxe2x80x94for instance, three colors of phosphor like red, green and bluexe2x80x94to the areas of the panel that were left free by the black matrix structure.
In producing a screen with a dotted structure, light from a point source is directed through the segmented lens and the shadow mask. This segmented lens comprises a rectangular array of differently inclined facets If the screen is illuminated through a stationary segmented lens, the images of consecutive facets will not fit as consecutive areas on the screen. This will cause dark and light lines, during the exposure process, in the areas where the images of two consecutive facets are disjunct or overlap, respectively. This phenomenon is normally referred to as facet marking. In order to obtain a substantially uniform illumination across the entire screen, the segmented lens is wobbled and drifted in oblique directions with respect to the rectangular array of facets. The wobble and drift directions are mutually nearly orthogonal, In this method the image of one facet is spread across a larger area so that the light and dark lines are faded to such an extent that facet marking is considerably reduced and even prevented.
In the currently used method of producing screens by exposure, the use of a point source for illuminating the screen daring exposure leads to a screen structure that closely resembles the mask structure. If, for instance, the mask has a structure of round apertures, the apertures in the black matrix will also be substantially round. For new designs of dotted-type tubes, it is often recognized that the currently used method has its drawbacks.
The structure of a dotted-type screen is, amongst others, determined by the horizontal and by the vertical pitch. In this context pitch means the distance between phosphor dots of the same color. In general, a small pitch is desired in order to obtain a good resolution of the screen. On the other hand the vertical pitch should be chosen to be such that the scan-moirxc3xa9 phenomenon is suppressed as much as possible. In order to fulfill these two requirements it appears that it is generally not possible to use a purely hexagonal screen structure. For instance, if the desired vertical pitch is increased with respect to the pitch corresponding to a hexagonal structure, the circular apertures in the black matrix lead to less electroluminescent material on the screen. As a consequence, the display device will have a lower luminance (light output).
In most commonly used CRTs, the screen is scanned in two directions. The line scan, being the higher frequency, is usually in the horizontal direction, while the frame scan, being the lower frequency and perpendicular to the line scan, is in the vertical direction. It is remarked that the frame direction is not per se the vertical direction. The frame direction in the vertical direction is not to be considered limitative.
The interaction between the mask and the consecutive lines causes the scan-moirxc3xa9 phenomenon. It is a disadvantage of the screens produced by means of the method mentioned in of the opening paragraph that a good moirxc3xa9 performance is mostly at the expense of the luminance.
It is an object of the invention to provide an improved method as compared with the method described in the opening paragraph, of producing a screen with a dotted structure for a color display device.
According to the invention, this object is realized with a method which is characterized in that said facets comprise light-refracting means having a base surface coinciding with the top surface of the facet and at least a first and a second light-refracting surface disposed at predetermined angles with respect to the base surface, thereby creating a number of virtual light sources corresponding to the number of light-refracting surfaces.
The invention is based on the recognition that, by providing each facet with light-refracting means having at least two light-refracting surfaces, the real point source viewed from the screenxe2x80x94is subdivided in to a number of virtual light sources equal to the number of light-refracting surfaces. The light-refracting means may be, for example, a prism structure.
It is to be noted that, for instance, British Patent Specification 1 577 503 discloses a rectangular structure of small prisms with two light-refracting surfaces. The purpose thereof is to create two virtual line-shaped light sources in order to be able to control the phosphor line width across the entire screen of a CRT. In contrast to the present invention, which is meant for CRTs with a dotted screen and a black matrix structure, the CRT described in said British Patent Specification is a tube with a line structure on the screen, originating. from a striped shadow mask and without a black matrix. By changing the distance between the two virtual light sources across the screen, a compromise can be achieved in this case between the phosphor line width and the phosphor adhesion on the screen. Although the use of prisms creating virtual light sources is known per se, the present invention describes a totally different measure compared to said British Patent Specification 1 577 503.
A preferred embodiment of the method according to the present invention is characterized in that the light-refracting surfaces are inclined in the frame direction in such a way that the virtual light sources are separated in said direction.
The inclination of the light-refracting surfaces in the frame direction causes the split-up in virtual light sources to be also in the frame direction. In this embodiment, the light-refracting surfaces cause the light spot on the screen to be elongated in the frame direction and, as a consequence, the apertures in the black matrix are elongated in the frame direction as well. With this measure, an improvement with respect to moirxc3xa9 and luminance is achieved. It is possible to optimize the pitch in the frame direction for moirxc3xa9. Moreover, the elongation in the frame direction of the apertures in the black matrix can be chosen in such a way that the amount of luminance that was lost as a consequence of the larger vertical pitch is at least compensated for.
Another drawback of the prior-art method is three exposure steps are necessary that for making the matrix structure, namely one for each color. Three apertures in the black matrix layer, commonly called a triplet, correspond to each aperture in the shadow mask. After the matrix has been applied, the three colors of electroluminescent material are deposited in the corresponding matrix apertures. As exposure of the matrix structure in three steps is time-consuming and requires expensive and complex equipment for exchanging the different exposure lenses that are needed for the patterns of the three colors, this is considered to be a drawback.
It is therefore another object of this invention to provide a simplified method of producing the black matrix structure of a screen with a dotted structure for color display devices.
An embodiment of the method according to the present invention is characterized in that the light-refracting means has three light-refracting surfaces, the two outer surfaces being inclined in the line direction in such a way that the virtual light sources are separated in said direction. In this embodiment, the point light source is split into three virtual light sources that are separated in the line direction. In this case, it is possible to make the prismatic action so strong that the distance between the virtual light sources becomes so large that the three resulting spots on the screen are disjunct to such an extent that they exactly form the three spots of one triplet. A triplet is the collection of three dots on the screen, each of them having a different color of electroluminescent material, obtained by exposure through the same mask aperture. Normally, the triplet is oriented parallel to the direction of the line scan, if the beams in the electron gun are oriented in the line direction.
The invention also relates to a screen of a color display device produced by means of the method according to the invention, as well as to a color display device provided with such a screen.